1. Technical Field of the Invention
This invention relates generally to integrated circuits and more particularly to excessive output current detection of such integrated circuits.
2. Description of Related Art
Integrated circuits (IC) are known to be used in a multitude of electronic devices and are required to provide sufficient output power to driver components coupled to the IC. For example, an audio processing IC (e.g., audio codec, MP3 player, etc.) has at least one output to drive headphones. To protect an IC from an overload and/or short circuit on such a high powered output, the IC includes a current limiting circuit.
FIG. 1 is an embodiment of a known output current limit circuit that includes an output drive transistor (Tout) and a temperature sensing circuit that is in the vicinity of the transistor. The output drive transistor provides an output current to a load via an IC pin. If the output current becomes excessive, the output drive transistor heats up, which is sensed by the temperature sensing circuit. When the temperature of the output drive transistor becomes too high, due an overload or a short, the temperature sense circuit provides an overload current signal to a processor, which disables the output drive transistor. While such a circuit provides overload protection, it does so at the cost of a temperature sensing circuit, which may not respond fast enough to avoid overloading the power rails.
FIG. 2 is a schematic block diagram of another known current sensing circuit. In this embodiment, a sense resistor (Rsense) is coupled in series with the load (Rload) off chip. The output transistor, which is on chip, drives the series combination of the resistive load and the sense resistor. An amplifier, or comparator, monitors the voltage across the sense resistor with respect to a reference voltage. When the voltage across the sense resistor exceeds a voltage reference, a current limit signal (Isense) is produced. While this provides overload protection, it does at the cost an additional sense pin for each output. Further, the sense resistor adds impedance to the output, which lowers the overall effeciency of the output due to its power consumption and reduces the voltage swing of the output.
FIG. 3 is another embodiment of a known current limiting circuit for an output of an integrated circuit. In this embodiment, a limiting resistor (Rlimit) is coupled in series with the resistive load (Rload). The limiting resistor is on-chip with the output transistor (Tout) and has an impedance substantially equal to the resistive load. An issue with this embodiment is the loss of voltage output swing for the resistive load as well as the inefficiency due to power consumption of the limiting resistor.
FIG. 4 is yet another embodiment of a known current limiting circuit. In this embodiment, the output transistor Tout drives the load resistance and the output current is mirrored to a sense circuit. The sense circuit includes a current mirroring transistor, a sense resistor and a comparator, or amplifier. In operation, the output current is mirrored by the mirroring transistor, where the mirrored current is provided to the sense resistor (Rsense). The sense resistor produces a sensed voltage based on the mirrored current, where the sense current is compared with a reference voltage. When the sensed voltage exceeds the reference voltage, an overload condition (Isense) exists. An issue with this embodiment of a current limiting circuit is that when the output voltage swings from rail-to-rail, the drain source voltage of the output transistor has a wide variation, which causes the mirroring transistor to produce an inaccurate mirroring current. Such inaccurate current mirroring cause unacceptable variations in the current limiting function.
Therefore, a need exists for a current threshold and/or limiting circuit that overcomes the drawbacks of previous current limiting and/or current sensing circuits.